Process for the electrolytic refining of lead



United tates Patent i PROCESS FOR THE ELECTROLYTIC REFINING 0F LEADGordon Henry Turner, Trail, British Columbia, Canada,

assignor to The Consolidated Mining and Smelting Company of CanadaLimited, Montreal, Quebec, Canada, a corporation of Canada No Drawing.Application April 3, 1957 Serial No. 650,303

Claims priority, application Canada April 10, 1956 4 Claims. (Cl.204-114) This invention relates to an improvement in the electrolyticrefining of lead by the Betts process. The invention is particularlydirected to providing a new and useful tion which contains from about 4to 10 percent lead as .lead fiuosilicate and from about to 12 percentfree hydrofluosilicic acid. The current density employed is usuallywithin the range of from about 12 to about 22 k amperes per square footof anode area.

The residence of the electrodes in the cell may vary over a relativelywide range depending on the nature and the amounts of the impuritiespresent in the impure lead and on operating factors such as thecomposition of the electrolyte, current density and the operationtemperature ofelectrolyte. A typical practice at one refinery has beento employ a three day cycle for the cathodes and a six day cycle for theanodes. Thus, two sets of cathodes are obtained from each set of anodes.The anodes are then removed, new electrodes are set and the cycle isrepeated.

A problem associated with the electrolytic refining of lead is that ofestablishing and maintaining a dense, smooth cathode deposit. It isknown that the lead tends to deposit on the cathode in a rough,irregular pattern with the formation of growths, known as peanuts andtrees. As deposition continues, these growths extend to and contact theanodes, cause short circuits, and reduce the efiiciency of the cell.Also, the rough surfaces of the cathodes tend to collect and retainsmall particles of impurities which are suspended in the electrolyte.

This problem is well known and many addition agents have been suggestedand used in an effort to overcome it. For example, small amounts of glueand mixtures of glue and goulac, a calcium lignin sulphonate, have beenemployed to improve the cathode deposit and overcome the problem ofirregular growths. These known addition agents have improved thecharacter of the cathode deposit to a certain limited extent but theproblem has not been completely overcome.

I have discovered that the addition to the cell electrolyte of a finiteamount of an extract prepared from the wood of western red cedar, eitheralone or in combination with a small amount of goulac, improves thequality and the structure of the cathode deposit.

The preparation of the addition agent of this invention is very simple.Western red cedar in the form of shavings, or chips, or sawdust, or inmore finely divided form, is leached with hot water, for example, at 90C. for 3 hours, to extract water soluble constituents, Wood 2,877,165Patented Mar. 10, 1959 cellulose and lignin normally are not soluble inhot water; other constituents are at least partially soluble. At the endof the leaching period, the solids are separated from the solution, forexample, by filtration. This filtrate may be evaporated to dryness andthe residue redissolved in water as required, or the filtrate may besubjected to partial evaporation, if desired, to give a solution ofconvenient strength to be added to the electrolyte.

The cedar extract can be used as an addition agent alone or it can beused in combination with other known addition agents such as goulac.Preferably, the extract is added to the electrolyte as an aqueoussolution of known strength.

Western red cedar, according to Canadian Woods, 1951, published by theForestry Branch, Forest Products Laboratories Division, Department ofResources and Development, Ottawa, Canada, is comprised of cellulose, 48percent, lignin 31.8 percent, pentosans 9.0 percent, and is 11 percentsoluble in hot water. The water soluble constituents provide theimproved results in the quality and physical characteristics of the leadcathodes obtained in the operation of the present invention.

I have found, in the extensive investigations to which this additionagent has been subjected, that the improvement is noticeable in the useof small amounts of extract, of the order of about 0.1 gram per litre ofelectrolyte, calculated on a dry basis, and the optimum results appearto be obtained within the range of from about 0.5 to about 2.0 grams perlitre when the addition agent is employed alone, or from about 0.1 toabout 2.0 grams per litre when it is used in combination with goulac.The goulac when used is added in amounts greater than 1 gram per litreand less than 7 grams per litre of electrolyte.

The investigations have also shown that the regular addition of 0.5pound of extract together with 1.0 pound of goulac per ton of leaddeposited gave very good results in a continuous process using a leadfluosilicate electrolyte containing from 0.1 to 2.0 grams per litre ofcedar extract and from 1.0 to 7.0 grams per litre of goulac.

These regular additions of cedar extract and goulac may vary between 0.1and 1.5 pounds of extract and 0.5 and 3.0 pounds of goulac per ton oflead to be deposited.

The improvement described herein has been shown by actual investigationsof the problem and is of course independent of theoreticalconsiderations. The smoothness of the deposit and the reduction ofcoarse edges appear to be related to cathode polarization. It appearsthat goulac is responsible for grain refinement at the surface of thecathode and that the red cedar extract provides a useful degree ofcathode polarization. Polarization tests indicate that the optimum rangeof concentrations of cedar extract is within the range set out above.

In a series of tests, it was found that an excellent lead cathodedeposit was obtained by an initial addition to the electrolyte of about0.5 gram of red cedar extract and 2.0 grams of goulac per litre ofelectrolyte followed by a daily addition of 0.05 gram of extract and 0.1gram of goulac per litre, the daily addition being equivalent to 0.5pound of extract and 1 pound of goulac per ton of cathode leaddeposited. The cathode deposit was smooth and dense.

The electrolyte used for these tests contained 8.0% lead as leadfluosilicate and 8.5% and 9.0% free hydrofluosilicic acid. The currentdensity was 22 amperes per square foot of anode surface and theelectrolyte temperature was maintained at 45 C. The experimental cellscontained .one anode and one cathode. The residence time for anodes andcathodes was three days.

The following series of examples further illustrates the improvedresults obtained in the use of the addition agent of the presentinvention. The electrolyte, in each instance, contained 8.0% lead and10% free hydrofluosilicic acid. The cells were operated at roomtemperature with a current density of 20 amperes per square foot. Ineach instance, the stated amount of addition agent or agents was addedat the start of each test and no further additions were made. Theresidence time of the cathodes was 48 hours.

Example Cedar Gonlac 48 hour Deposit, 20 amps. per Deposit No. Extract(g. p. 1.) sq. ft. Room Temp. (20 0.) Rating 1 0 Rough,wires, notadherent Poor. 2 0.1 0 Hard, rough, long needles, Do.

no shorting. 3 0. 0 Hard, rough, coarse crystals, Good.

good edges. 4 2.0 0 Hard, smooth, few bumps, Do.

good edges. 5 5.0 0 Hard, smooth, very bumpy, Poor.

wires on edges.

0.0 1 Hard, rough, small bumps, Fair poor edges. 0.1 1 Hard, smooth, fewbumps, Do.

fair edges. 0. 5 1 Hard, smooth, very bumpy, Do.

good edges. 2.0 1 Hard, smooth, few bumps, Do.

some wire, good edges. 5.0 1 Hard, smooth, bumps and Poor.

wire, badly warped.

11 0.0 4 Hard, smooth,fairedges Fair. 12 0.1 4 Hard, smooth, fair togood Good edges. 13 0.5 4 Hard, smooth, few bumps, Do.

good edges. 14 2.0 4 Hard, smooth, few bumps, Do.

very good edges. 15 5.0 4 Hard, smooth, bumps and Poor.

wires, badly warped.

10 0. 0 7 Hard, smooth, poor edges Fair. 17 0.1 7 Hard, smooth, fair togood Good edges. 18 0.5 7 Hard, smooth, few bumps, Do.

good edges. 19 2.0 7 Head, smooth, bumpy, good Do.

e ges. 20 5.0 7 Hard, rough, bumpy, badly Poor.

warped.

Example 1 illustrates the type of deposit obtained without the use of anaddition agent. Examples 3 and 4 illustrate that a good deposit isobtained by the addition of 0.5 and 2.0 grams of cedar extract withoutgoulac. The remainder of the examples illustrate the results obtainedusing varying amounts of red cedar extract and goulac. In the operationof the process it is preferred to use a combination of red cedar extractand goulac.

The improved addition agent of the present invention possesses a numberof important advantages. It is readily available and relativelyinexpensive to produce. In use, it facilitates the electrodeposition ofa smooth and dense cathode deposit.

What I claim as new and desire to protect by Letters Patent of theUnited States is:

1. In the electrolytic refining of lead by the electrodeposition of leadfrom an aqueous, lead fluosilicatehydrofluosilicic acid electrolyte, theimprovement which comprises the addition to the electrolyte of the watersoluble extract obtained by hot water leaching of the wood of westernred cedar, the extract being added to provide from 0.1 to about 2 gramsof extract, calculated on a dry basis, per litre of electrolyte.

2. The process according to claim 1 in which the electrolyte containsfrom about 1 to about 7 grams of goulac per litre of electrolyte.

3. In the electrolytic refining of lead by the electrodeposition of leadfrom an aqueous, lead fluosilicatehydrofluosilicic acid electrolyte, theimprovement which comprises providing in the electrolyte, per ton oflead electrodeposited therefrom, from about 0.5 to about 3 pounds ofgoulac and from about 0.1 to about 1.5 pounds, calculated on a drybasis, of the water soluble extract obtained by hot water leaching ofthe wood of western red cedar.

4. In the electrolytic refining of lead by the electrodeposition of leadfrom an aqueous, lead fiuosilicatehydrofluosilicic acid electrolyte, theimprovement which comprises providing in the electrolyte, per ton oflead electrodeposited therefrom, from about 0.5 to about 3 pounds ofgoulac and from about 0.1 to about 1.5 pounds, calculated on a drybasis, of the Water soluble extract obtained by hot water leaching ofthe wood of western red cedar, said electrolyte containing initiallyfrom 1 to 7 grams of goulac and from 0.1 to 2 grams of said watersoluble extract per litre of electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS1,544,726 Colcord July 7, 1925 2,664,393 Mathers et a1. Dec. 29, 19532,827,410 Smyers et al. Mar. 18, .1958

OTHER REFERENCES Mathers et al.: Transactions Electrochem. Society, Vol.21 (1912), pp. 313-316.

Mathers: Transactions Electrochem. Society, vol. 26 (1914 pp. 99-107.

1. IN THE ELECTROLYTIC REFINING OF LEAD BY THE ELECTRODEPOSITION OF LEADFROM AN AQUEOUS LEAD FLOUSILICATEHYDROFLUOSILICIC ACID ELECTROLYTE, THEIMPROVEMENT WHICH COMPRISES THE ADDITION TO THE ELECTROLYTE OF THE WATERSOLUBLE EXTRACT OBTAINED BY HOT WATER LEACHING OF THE WOOD OF WESTERNRED CEDAR, THE EXTRACT BEING ADDED TO PROVIDE FROM 0.1 TO ABOUT 2 GRAMSOF EXTRACT, CALCULTED ON A DRY BASIS, PER LITRE OF ELECTROLYTE.